This invention relates to an easily drillable, non-metallic, non-rotating plug set for use in well cementing operations.
Typically, at the beginning of the cementing job in rotary drilled wells, the casing and hole are filled with drilling mud. In many areas, to reduce contamination on the interface between the mud and cement a bottom plug is pumped ahead of the cement slurry. The bottom plug is typically constructed having five wipers of elastomeric material thereon to wipe the casing of drilling mud thereby separate the drilling mud from the cement slurry. When this plug reaches the float collar at the bottom of the casing string, a fluid pressure differential created across the plug ruptures a rubber diaphragm at the top of the plug and allows the cement slurry to proceed down the casing through the plug and floating equipment and up the annular space between the pipe and the well bore.
When all of the cement has been placed, a top cementing plug having typically five wipers thereon is released from the plug container. The top plug's function is to follow the cement and is designed to reduce the possibility of any contamination or channeling of the cement slurry with the drilling mud that is used to displace the cement column down the casing and into the annular space between the casing and the well bore. The top cementing plug is typically solid in construction and the design is such that when it reaches the bottom cementing plug at the float collar, the top cementing plug causes a shut off of fluids being pumped into the casing.
The landing of the top plug will lessen the possibility of any further displacement of the cement slurry and provides a better quality of cement slurry around the bottom of the casing where a good cement bond to the casing is required.
Typical prior art cementing plug sets which are used in well cementing operations have been constructed of wood, plastic or aluminum with an elastomeric covering thereon to form wipers which contact the well casing to be cemented in the well bore.
Previously, the plug set and the residual cement in the casing cemented in the well bore were removed by drilling operations using tooth type rock bits. The teeth on the rock bit proved effective in the drilling of the plug set, even though the individual plugs of the plug set were free to rotate with respect to each other and the floating equipment installed in the casing cemented in the well bore.
Recently, with the advent of polycrystalline diamond compact (PDC) drill bits to drill out the plug set and the residual cement in the casing cemented in the well bore it has become necessary to use a non-rotating plug set during the well casing cementing process to facilitate the drilling of the plug set, floating equipment and residual cement. For whatever reasons, the teeth on the PDC drill bit do not as effectively drill through the conventional plug set used in casing cementing operations as the conventional tooth-type rock bit. However, with the use of a non-rotating plug set in casing cementing operations, the PDC drill bit can drill through the plug set, floating equipment and residual cement in time periods comparable to that of conventional tooth-type rock bits.